In large bowl, with mixer at high speed, beat egg whites until soft peaks form. Sprinkle in 1/4 cup sugar, 1 tablespoon at a time, beating until whites stand in stiff, glossy peaks when beaters are lifted. Set whites aside.

In another large bowl, using same beaters, with mixer at high speed, beat egg yolks and remaining 1/2 cup sugar 5 minutes or until thick and very pale yellow. With rubber spatula, fold in oil until incorporated. Fold in half of egg whites, then fold in cocoa mixture until no dry streaks remain. Fold in remaining whites just until no white streaks remain.

Gently spread batter evenly in prepared pan. Bake 10 to 12 minutes or until top of cake springs back when lightly pressed with finger. Cool cake in pan on wire rack 10 minutes.

Move pan from rack to work surface. Sift cocoa onto top of cake. Spread a clean lint-free kitchen towel over cake, then place the wire rack upside down on top of towel. Invert rack, towel, cake, and pan together. Remove pan, then carefully peel off parchment paper. Starting from a long side, loosely roll up cake with towel inside, jelly-roll fashion. Place roll, seam side down, on wire rack to cool completely, about 1 hour.

Prepare Caramel Filling: In small bowl, sprinkle gelatin over water. Let stand 5 minutes to soften gelatin. Microwave gelatin mixture on High 15 seconds or until just dissolved. Let cool to room temperature. In medium bowl, with mixer at medium speed, beat cream until soft peaks form when beaters are lifted. With mixer running, add gelatin mixture in slow, steady stream. Continue mixing until stiff peaks form. In large bowl, stir dulce de leche and one-third of whipped cream until well combined. Gently fold in remaining cream until combined.

Gently unroll cooled cake. With metal spatula, spread filling evenly over cake almost to edges. Starting from same long side, roll cake without towel. (Cake may crack slightly as you roll it.) Wrap cake roll tightly in plastic wrap and refrigerate at least 1 hour and up to 1 day.

Cut diagonal slice (1 1/2 inches thick) from each end of cake roll, and reserve. Transfer cake roll to large serving platter. Reserve 1/4 cup frosting; spread remaining frosting all over cake. Place reserved end pieces on side of roll to resemble cut branches, cut sides facing out. Spread reserved frosting over outside of branches, leaving cut sides exposed. With damp paper towel, wipe platter clean. Cake can be covered loosely with plastic wrap and refrigerated up to overnight.

When ready to serve, garnish platter with grated chocolate and Meringue Mushrooms, if you like. If cake has been refrigerated, let stand at room temperature 15 to 20 minutes before serving for easier slicing.